FTP on the Go Support about Images
Image Stored in the Photos app
Any photos that are stored onto your iPhone or iPod touch by iTunes or the Camera are automatically optimized to display faster and take less space. This means they're converted to a smaller size (the exact size depends on the device, iPad and iPhone 4 images are larger). If FTP On The Go uploads pictures that have been imported, they'll be this size. Photos taken with the camera from within FTP On The Go are at the camera's resolution.
For most people, that's good, so the space on the iPhone isn't filled by big image files that are several MB each. But for photographers, you may want the original image size and quality....
Camera Connection Kit
If you import photos using the Camera Connection Kit, they are stored at higher resolutions. Make sure that you use the "Photo Library" from the "Saved Files" tab to access your images. This gives direct access to the image files, at full resolution.
Note that really big images can be larger in size than the data an iPhone program is allowed to use total! The exact size will depend on your iPhone/iPod touch/iPad device. Earlier models have less memory and will have more trouble with large image files. (And it may depend on things like how recently your iPhone was rebooted too!) You'll of course be able to store, upload, or download them--but may not be able to view them on the iPhone.
FTP On The Go also resize photos, either as you upload, or making a resized copy in your Saved Files. Again a really big photo may not resize because of the iPhone's memory limits.
But you're saying "My iPhone has 8 or 16GB or 32GB, how can it be out of memory?"
Those numbers are like the Hard Drive on your computer. The other number, that isn't really mentioned as much is equivalent to the RAM on your computer: 512MB for the iPhone 4, 256MB for the iPhone 3GS, 128MB for the iPhone 3G, etc. For everything; the operating system, processes waiting to see if you get a phone call or text message, playing music, etc. Newer iPhones, the 3GS and 4 have much more memory, so will be able to better handle large images than the original iPhone or the 3G.
Since the iPhone is #1 a phone, if a program starts to use too much memory, the operating system will kill the program to keep the core functions intact. That's why sometimes an app will close, but the music keeps playing and the phone keeps ringing.
Opening a really big image (many MB in size) can easily use up all the memory FTP On The Go is allowed, and can end up with it being terminated--or just not displaying the image correctly.