egg is an expert in xara animation, I am not, I use another program, but one cardinal rule I will never deviate from is 'always work in whole pixels' - your final animation is going to be sized in whole pixels, so you must make sure you crop your images to whole pixels and in the correct aspect ratio

obviously, if the image[s] you are using are not in the same aspect ratio as your final video size you will need to have white space either in x or y; if you are making an animation that is not a standard video size maybe not, but you must still have your images in whole pixels

your screen capture should be in whole pixels and you should keep it that way - if somewhere along the way, either by cropping or whatever, your image has one dimension that is in pixel fractions you are going to have to either remake it, or, pad it to whole pixels by placing it over a white rectangle of the correct whole pixels size and making a bitmap copy [thereby introducing some white space], or recropping it at a whole pixel size [thereby losing a small part of it]

this is particularly important in xara whose reporting of image size is only as accurate as the number of decimal points your windows OS is set to; a small variation can completely throw an animation, as you noted earlier....