One reason you don't want to use JPG format is your photo degrades each time its processed. First, you take what I'm assuming is an original Jpeg photo into Photoshop(PS), which needs to be decompressed before PS can resize or add any effects. Saving it once again in JPG, can further degrade and possibly add visible digital artifacts depending on your settings. In fact, with each compression/decompression cycle introduced, more of your photo's info is generally discarded. So not available when its decompressed.
The same resized JPG photo imported into your video editor, must once again be decompressed before the editor can process it in any way. This is the reason you should opt for a lossless format early in the process to avoid quality loss of your source edit materials.
Note that most video editors can resize photos or graphics. So may not be necessary to use a separate image editor beforehand.
