Hello there.
I looked at the ART in Red theme and indeed there is an issue with the album art. It's an old theme (early 2010) so maybe at the time (Rockbox release 3.4) it worked properly, I don't know, but for sure it can't with the current skin engine since there are only 2 layers (backdrop layer + foreground layer) and you're not supposed to draw several images on top of each other on the foreground layer. The guy who made this theme used ARTinRed-wps.bmp for background, that's OK but used both cover.bmp and the album art on the foreground layer. The skin engine doesn't know which one is supposed to be on top of the other, that's why you see the album art flickering: sometimes the album art is on top, sometimes cover.bmp.
The first workaround I was thinking about is to merge ARTinRed-wps.bmp and cover.bmp into a new bmp file, and then display this background as conditionnal (depending on play or pause playback status). It would be something like:
%xl(back_without_cover,ARTinRed-wps.bmp,0,0)
%xl(back_with_cover,ARTinRed-wps2.bmp,0,0)
%V(0,0,-,-,-)%VB
%?mp<%xd(back_without_cover)|%xd(back_with_cover)|%xd(back_without_cover)>
But that would require editing the WPS code, which is not so easy since it looks terrible and confusing (sorry for the Author :S)
Then I thought about a much easier workaround: edit cover.bmp and replace the background with transparency, so that the album art would always be visible, even if cover.bmp is on top.
So just replace the current cover.bmp in
.rockbox\wps\ARTinRed by the one I put in attachment and that should work

PS: as bmp files are not allowed in attachment, I renamed it to jpg instead. So just rename it to bmp again.