The Bais Yossef in responsa Avkat Rochel (Chapter 63) writes lengthily about this matter and writes that one should not make forms or images of lions and the like on the curtain of the Aron Kodesh or on the Aron Kodesh itself.
Ohr Zarua (Masechet Avodah Zara, Chapter 203) writes that there was an incident in Cologne where images of lions were drawn on the windows of the synagogue and Rabbeinu Elyakim ruled that they must be removed based on the verse, “You shall not make for yourself a statue or any image.” Furthermore, when the congregation prays and bows while facing these images, it seems as if they are bowing to the images and this is forbidden. The Ohr Zarua adds: “I remember that when I was younger, they would draw forms of birds and other animals in the synagogue and I ruled that this is forbidden, for this would cause people to pay attention to the beauty of the images as opposed to concentrating on their prayer.” He brings sources to support his view.
Chida (in his Sefer Shiyurei Beracha, Yoreh De’ah, Chapter 141) quotes a certain Gaon who writes that it is obvious that one may not allow such images of lions in synagogues or Batei Midrash (Houses of Study) where people pray on a regular basis. He adds that anyone who rules leniently on this matter will have to give reckoning before Hashem for this.
Yechave Da’at, (Volume 3, Chapter 62) that it is forbidden to hang a curtain embroidered with images of lions on the Aron Kodesh. He adds that rabbis in Israel and all over the world for that matter must raise public awareness about this issue, convince the caretakers of synagogues to remove such curtains and other such forms around the synagogue, and to no longer produce such curtains with these images.
A slightly lenient opinion from Ben Ish Chai in Parashat Yitro, distinguishes in this regard between pictures drawn on the wall itself, and images that protrude from the wall. In the case of illustrations drawn on the wall, the Ben Ish Hai writes, one may pray facing the wall provided that he closes the eyes to demonstrate that he does not intend to worship the pictures. When it comes, however, to images that protrude from the wall, the Ben Ish Hai forbids praying facing toward the wall, even if one closes his eyes.
Background discussion:
Mador Shechina: Angels; lions; ox; eagle, Man (Michaber 141/4; Abayey in Avodas Kochavim 43b; Omitted in Rambam Hilchos Avodas Kochavim 3/11, see Kesef Mishneh there):
It is forbidden to form the creations that are found on the chambers of the Shechina[The reason: As the verse states “Lo Sasun Iti” which refers to those creatures that live with Me [Hashem]. [Shach 141/20; Rashi ibid] [the Divine chariot] such as:
a) The four faces [on the chariot, which are the lion/ox/eagle/man[Shach 141/20 in second explanation; Rashi ibid] or lion/baby/eagle/adult[Shach 141/20
Face of an ox versus baby: The Gemara Chagiga 12b states that Yichezkal came along and switched the face of an ox for the face of a Keruv, which would imply that the face of an ox is no longer part of the chariot.
[Shach 141/20 in his first explanation]
However, in truth, one can say that the face of the ox still remains part of the prohibition, being that it was on the chariot at the time that verse of prohibition was stated. [Shach 141/10 in his second explanation] At the same time however it is also forbidden to form the face of a baby, together with the other three faces of the adult/lion/eagle together. [Shach ibid]] all in one picture [as the face of a single animal[Shach 141/20, 21, 30; Taz 141/7; Rashi ibid explains “for a single animal”, meaning that a single animal may not contain these four faces.]. [It is however permitted to form these faces individually, with exception to the human, which may not be made even individually, as will be explained.[Taz 141/7; Shach 141/21, 30; Semak 160; Ran] It is likewise permitted to form two or three of these faces, with exception to the human face in which some Poskim are stringent, as will be explained. The above prohibition only applies to making a sculpture of the entire body together with the face, however to just make a head with the four faces is permitted.[Taz ibid; See Michaber 141/7
The reason: As in the chambers of the Shechina the four faces were complete with bodies. This is also proven from the fact the Ran and Semak ibid question why the Talmud implies that only four faces together are forbidden while in truth even man alone is forbidden, and they do not answer that here the prohibition refers to just the face, which is permitted by man. This proves that the entire case of the Talmud referred to making a body with the four faces and hence their question applied. [Taz ibid]
Other opinions: Some Poskim rule the faces of the chariot did not have complete bodies and hence the prohibition applies even to simply making a headwhich contains the four faces. [Maharit Y.D. 2/35 in implication of question of Ran ibid who did not answer that a mere head of a human is allowed; brought in Gilyon Maharsha 141/7]
If one found the sculpture of a human and then added to it the other three faces he transgresses the above prohibition.[Taz 141/7; Shach 141/21; Semak 160; Ran; Tosafus Avoda Zara 43a and Rosh Hashana 24]]
b) Angels such as Serafim, Ofanim, [Chayos Hakodesh[Avoda Zara 43b; VeTzaruch Iyun why Chayos Hakodesh was not mentioned by Michaber ibid]] and Malachei Hashareis.[Michaber and Gemara ibid] [This refers to a human like figure with wings.[Taz ibid]