To help create test-images, I created this little FireFox Add-on. The Counting Images add-on starts with one click and can be used to create an image of a custom size.
For example: if you need a 300x250 MPU advert image - just enter 300 and 250 into the panel and click Create Image. To download the image, just click on it - as you would a a link and choose Save.
The image files are named widthxheight.png, and include markings to help identify if they have been truncated e.g.:
What could go wrong? Well a good example is very thing and tall images. The image edge might actually truncate the text specifiying its height e.g.:
The image here is 30x1001 but the narrowness means the visible text is 30x100.