It is not the size of the file on disk, but the size of the file loaded into memory. If your files are .jpg they are compressed and use a lot more memory when loaded. What is the pixel size of the images? 1024 x 600, etc.
Also, each time you exit your app, it is still in memory unless you used: ExitApplication which will terminate it. This is not recommended but you can add it to check things. You need to trap the backkey and force it to call the code ExitApplication and try your load and exit and see if your error goes away. If it does, it is most likely a memory issue.