I was having the same exact problem [the album art doesn’t show up in my Sansa], but I figured out a way to fix it:
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I use MSC mode
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Convert all of the ID3 tags to v2.3 [some of my songs were in v2.2 and they wouldn’t show; now all the v2.3 show album art]
This can be done in iTunes: Right click on song > choose “Convert ID3 Tags…” > Under “ID3 Tag Version”, choose 2.3
- Copy all album art JPG images to the Sansa
This works 100% on all of my songs