The app only provides common image support same as the iOS system, any image viewable with iOS's native apps, such as Photo, Safari, could be imported as image overlay to the app.
The support of image overlay is very limited, although we support custom boundary setting and rotation, but not geo-specified, which means all metadata, such as Georeferencing info, CRS, GCPs of the images would never be loaded. There is no difference for using a PNG than a GeoTiff.
At last, about the performance. The image overlay is designed for small or normal images, for example, images less than 50MB. For a 3GB image, the loading and tiling(drawing) would be extremely slow, although the app support caches to make it faster for drawing the same region at the same zoom level, but it is not suggested to use in that way.
For huge images, > 500MB for example, it is suggested to convert it to pre-tiled format before importing to this app. With some free tools, you can georeferencing and tile down a huge image into tiles and save it as a *.mbtiles file, following article shows how:
http://www.duweis.com/support/viewdoc.h ... bt&from=mp