Do-it requires your images to be certain sizes. There are lots of different tools you can use to resize images, this guide will show you one easy way.
1) Go to http://apps.pixlr.com/editor/
2) Click on 'Open image from computer', and select your image
3) From the top menu, click on Image, and from the drop down menu select 'Canvas Size'
4) Enter the number of pixels you want your image to be. Logos should be 500x500; header images should be 1170x391 or 2340x782 - see guidance here.
Also, remember to click on the middle box to centre the picture in the middle of the image.
5) Click File, and Save the image as a new file - remember to name it clearly, so you know it is the right image to use on Do-it.