If you have Paint on your pc, then resizing your photos is dead easy. Locate the image you wish to resize on your pc, right click on it and then left click on 'Open with...' scroll down the list it will bring up and select Paint.
Once the image is open in Paint, look up at the top left of the window and select 'Resize'...left click on that, and then choose how much you wish to resize the image (I normally go for 50%) Left click on your chosen size, select 'save as', and Paint will save the resized image for you, normally in Downloads or in Pictures on your pc.
Once you've done that, locate the image on your pc and just make sure that the image is less than 300kb and it will be fine to upload to Mayhem. (just right click on 'Properties' and it will display the image size for you) If it's still too big, then reopen in in Paint and resize it a bit more, you'll soon get the hang of it.
Rich