I have just uploaded one fine. Are you sure they are small enough? Mumsnet prefers photos to be smaller before uploading than other sites. I usually resize mine to 300 x 400 first. I do it in Photoshop but there are lots of free programs which will do it for you - there are links on the upload photo page.
They sound like they are the right format, yes, the format means whether they are .jpg etc. You probably need to make them smaller though. Will copy and paste the MN instructions on how to because I am rubbish at explaining things quickly
Your image files should be JPEG images (they'll usually have a .jpg extension) Many digital cameras take very high-resolution images and store them as very large files. Even on broadband, these files can take over two minutes each to reach our server. The server won't accept files that are too large. It may be easier and quicker for you to make reduced-size copies of your photos before you upload them to us. Your digital camera may well have come with some software for this purpose. Check if your camera program has a sharing or emailing option. These will normally make smaller copies of your pictures without affecting the original.
If not, there are many free programs available that will shrink images. If you are using Microsoft Windows, Microsoft makes available a free product - the "Image Resizer Power Toy" for this purpose. To use it, you can go to Microsoft PowerToys and download the Image Resizer from the list of items on the right side of the page. It's very easy to use. If you'd rather not install new software, Windows XP lets you shrink images to send in an email. Microsoft has instructions on how to use this facility on this page. You can save the attachments this procedure creates as a new file to upload to us.
Similar utilities are available for other operating systems. For example, new Apple Macs come bundled with iPhoto. This can shrink images and also do many other types of image manipulation. To export a picture in a smaller size from iPhoto, click it in your library then choose Share then Export from the menu, and follow the instructions on the screen to save a copy with a smaller size. In the "scale images no larger than" box, replace the higher number with 600 and click export to save a shrunken copy of your picture. This should create a file small enough to upload to us.