720 x 1756 = 4.8 MB (uncompressed bitmap in memory)
If you have even 10 of those you might hit the memory limit.
And remember each Android version came with different memory management (so Froyo etc will hardly load 3 of those images)
To make sure your bitmaps are cleared, you need to make sure there are no references to it anymore.
i.e. all imageview.bitmap = null , and the bitmap is not stored in a list or variable anywhere else.
It is a little weird though, while you remove all views, it still comes back, for some reason they are not being cleared.
Which phone/device/android versoin you are running on?