Updated with screenshot & with Windows version in title.
Shotcut project video format on Automatic and project initially loaded with an .mp4 1280x720.
Using externally created image files for title cards and overlays, etc. then D&D in to Shotcut playlist. Playlist displays item data fields In, Duration, Start. Image files generated at 1280x720. For title cards initially used .png.
.png Duration set by Shotcut at 19884:06:28:07!!! Preview player timeline shows this as well. OK, double-click .png item in Play list and select Properties tab, confirmed title of Properties panel as the .png file. OK, Duration seems editable, so set to 5:0 then [RETURN} and Shotcut updates Duration to 00:00:05:00. Good. Select Playlist tab. Playlist Duration still shows 19884:06:28:07 as well as Preview timeline. So, item properties changes in Properties tab do not stick. Attempt to use Preview timeline to make a 5-sec cut with no joy. Dragging in to a Track means Shotcut then tries to make a Track of 19,884 hours in length. Have to use Windows Task Manager to kill Shotcut.
For grins try .tif. Now, Playlist Duration shows 19884:06:28:07 and Start as -19884:-2:-8:-15. Yes, with the minus signs. Also, Preview image is vertically squashed by ~30%.
So, try .jpg. Now, Playlist Duration shows 00:00:00:01 - a single frame which is annoying for initial track work not unreasonable. Good! Oh, beans, the Preview image is a white card with no text.
So, try .gif. Get same Playlist Duration as .png of 19884:06:28:07.
Verified all image files using GIMP.
Some other video editors provide for a user setting of default Duration for image files, with shipping default of 10 secs.
Still-image import and properties are broken.