I would like to edit a simple PSD with a smart object like this one. I only need to replace the smart object with a screenshot of an app and export the image as a PNG or JPG. Is there an alternative to Photoshop available that can do this? (I'm using Windows 8.1 and I don't need any other of the advanced Photoshop features)
Probably you are limiting your options here. You are limiting to PSD files and Smart objects. And that does not give you much choices.
If you are open to another options:
Use any program that uses layers (or at least objects)
1) There you can use gimp for example.
- Resample your png or jpg and paste it into your background.
2) Use any layout program. Scribus, page plus (Or corel or ilustrator, which are again too powerfull for your needs.)
Make a layout with a linked image.
You now can update that linked image and export your file.