If you are trying to load photos in Matrix and they are too large (generally 20mb+ per photo), there is a free program called PixResizer that easily allows you to shrink the photos down so Matrix will accept them.
You can get PixResizer for free here.
Here is what the PixResizer screen looks like:
The tabs at the top allow you to work with an individual file or a group of files at once. If you have a bunch of photos to resize, put them in a folder on your desktop, My Documents, etc. and use the Work with multiple files tab.
1. Click Source to select the file or folder you want to work with.
2. Click Destination to choose where you want the resized photos to be saved. If working with multiple photos, you may want to create a new folder on your desktop for this.
3. Drag the slider until you're at 1024- Matrix wants photos with dimensions of at least 1024x768.
4. Make sure the file format is JPEG (this is typically already selected when you open the program).
5. Click Save Pictures. It will save the newly sized photos in the Destination you chose in step 2.
You're all done. You can now go into Matrix and load the photo(s) from your Destination folder.