Can I set a background picture with a higher-resolution than created by default?
The answer is 'yes and no.' We'll address these in the opposite order:
No: the program currently will automatically size the background photo you choose to fit the screen, and in the process, drop any special resolution the original photo has. There is no setting to tell SimsUshare to use a higher-resolution photo.
Yes: you can manually substitute a higher-resolution photo into the sim, once SimsUshare has brought the photo in. Here are the steps:
Choose the original photo in SimsUshare to have it step down the resolution
Look in the sim folder to see the name that SimsUshare gave it (something like "BK0.jpg", "BK1.jpg", etc.). Let's say that photo is BK2.jpg.
Locate your original photo, and copy it in the sim folder, overwriting BK2.jpg.
Do this for all the background photos you want to resize.
Open SimsUshare and edit the simulation in question.
Go to the location which has the picture you wanted to size.
Go to the menu and choose "Adjust Background Photo".
Use the slider that appears on the bottom left to shrink your photo to the correct size.
If you have an image-editing program like Photoshop, or use something online, you can size your photos outside SimsUshare and adjust the "scaleX" and "scaleY" properties of the photo, inside the simdef.xml file (your sim definition file), but this requires more knowledge of using the editing program and potentially editing XML files.