You’ll probably spend most of your time editing already existing images, especially photographs that have been scanned, rather than designing new images.
In this case, select Open from the File menu and use the "Open" dialog to locate and open the required file.
The image file will appear in the Photopaint window as a file of the type in which it was originally opened. Since there are some file types which do not preserve any layers you might have added, you may choose to use Save As from the File menu to convert the file into a Photopaint image file (that is, with extension .apx). This guarantees that any layers you add will be preserved, rather than being flattened onto the original image’s background (where they cannot be recovered on a subsequent opening of the file). Photopaint files will, of course, require more memory to preserve layers.
Once you have opened the file, the image will appear in the image window. At first it will be displayed using the actual pixel size and therefore might be larger than the image window. If you would like to see the whole of the image, select Fit on Screen from the View menu.
The image will appear on a layer called Background, unlike a new image where the background layer is called Level 1 (see Layers for some notes on the difference between the two).
At this point you can immediately begin to use the paint and selection tools on the image. However, you probably don’t want to change the background image itself, so it is best to create a duplicate layer, hide the background and proceed to edit the duplicate or create further layers. All this is explained in the section on layers (see Layers).
Another option is to create a duplicate of the image file, using Duplicate in the Image menu. You can then work with the duplicate and leave the original untouched. Whatever you do it is highly recommended that you have at least one untouched version of the original file.