Once the images are saved to the computer. The way I resize, lazy I am, is using Powerpoint (PP).
Open powerpoint new presentation. Using insert picture, I insert the picture. Drag the picture out to fill the slide, or make a photo montage, and sometimes label the image on the slide. Then in PP, Save as jpeg. When prompted, click this slide only, name it and decide where to save it. This Jpeg file will upload on to Mayhem without any fuss.