Firstly I suggest that you post messages about FTM Mac on the FTM Mac board not the FTM Windows board. Reason being if it is really an FTM Mac problem you are likely to get a better response.http://boards.ancestry.com/topics.software.ftmmac/mb.ashx
But your question really is about images.
Effectively you should only resize the image once if you want to preserve its clarity especially if you are using a lossy format e.g. JPG. Often the images that you are capturing ar essentially very low resolution JPG images optimised for the web i.e. small file size and low quality. If you then start you resize them, you rapidly lose clarity and get a pixelated and fuzzy mess. A bit like making a photocopy of a photocopy of a photocopy etc.
I would suggest that where you can adjust the screen size before capturing the image so that you don't have to resize it.
If you really need to resize it, I suggest that you save the file outside FTM in a non lossy format, e.g. TIF or PNG and manipulate it in your photo edit software in these formats before linking it to FTM. It won't be ideal but it gives you a bit more to play with.
Have a look at some articles about this subject of resizing in Google and perhaps adjust how you deal with these matters.