One of the attached sprites (sho1) is working fine, but if I add the other sprite (aco1) to sprites.pk3, the game doesn't display anything. It's definitely the image file itself, tested it by using the same file name for either image in sprites.pk3. Any ideas?

Problem solved: <!-- m --><a class="postlink" href="http://forum.zdoom.org/viewtopic.php?f=3&t=36980">http://forum.zdoom.org/viewtopic.php?f=3&t=36980</a><!-- m -->

In SLADE3 you can: right-click your graphic > choose Gfx > Modify Gfx Offsets. Then in the popup you can select Automatic Offsets, Monster, and click OK.