There’s no difference in how you change the length of a layer in AE7.
If you can’t extend the length of a still image, what you might have is not a still image, but an image sequence. Check the Type column for the image file in the Project window. AE7 likes to turn on the “image sequence” checkbox in the Import dialog for any image it detects could be part of a sequence (ie., the folder contains numbered files), or you could also have turned on the checkbox for a single image. Re-import the single file with the checkbox off and you should be fine.
BTW, to change the length of still images in a comp (regardless of their format), go to the Preferences > Import, and set the length in the Still Footage area.