Hi Milene', 

you can adjust the default duration of still images in Preferences Panel (F5) in TIMELINE CONFIGURATION >> DEFAULT STILL IMAGES DURATION. This parameter can be set in hours, minutes, seconds and frames. The main problem you can get if the source frames have a size higher of the 1920x1080 they will use an huge quantity of memory and you can get out of memory errors if you scrub it quickly.

Generally for stop motion each picture lasts a frame so to avoid to use an amazing quantity of memory you can import your sequence as a virtual single clip: ADD AS IMAGE SEQUENCE (right click on Repository Window) but your sequence of BMP, JPG or TGA files have to use an accurate syntax like this  "FILENAME_0000X.JPG". (BASENAME +UNDERSCORE + 5 Digits + dot extension)

ivsEdits has been designed for video, that is usual 4:3 or 16:9 but you can force it to make squared videos cropping the output to match the max Y size (2160) making 2160x2160 videos  setting output display/pixel ratio to 1:1. Usual Crop X offset value for a 3840x2160 project is 840 pixels for each side to crop, see the picture above. You could generate video with greater resolution thanks to output upscaling but you have consider that the mixer engine can work actually at max size of 4096x2160.  the bitrate I have used in this picture sample is too much low, please increase it with 20000 or higher value.