Reduce canvase size to the same size as image
Photoshop can be too clever.
Best bet it to start with the original image, and if you want the whole thing, just resize image.
If you want part of the original, select the bit you want, then ctrl C it to copy
Open a new document (it should be the same size as the bit you just copied) and paste.
Then resize the new image and "save a copy as" to get jpg/gif