There are several ways to create BeOS icons from image files. Three methods are described here.

Via Icon-o-Matic

Double-click the icon well in any FileType panel. Once open, just drag any image for which you have an installed Translator into the main image area and it will be scaled appropriately. Then click the mini-icon well and drag the image in again, to make sure you’ve got both large and small icon sizes saved. Do a little touch-up if necessary, save, and you’re all set.

Via Thumbnail

Install a copy of Thorsten Seitz‘ excellent ThumbNail. Run any image for which you have an installed Translator through ThumbNail and it will create an icon from that image, then store that icon in that file’s icon attributes.

Image file icons created with Thumbnail

Via Photoshop

In Photoshop for MacOS or Windows, size your image to exactly 32×32 pixels and save it in .RAW format. Resize it again in 16×16 format and save under a different name. Move both raw files to your BeOS machine.
Create a new file, right-click it and choose Add-Ons | FileType. When the dialog pops up, drag your large image into icon field. It should change to the new icon. Then drag in the small version. You won’t get any feedback here, but it will have also been imported (you need the small version for Tracker list views).