You can configure a variety of editors to work with ACDSee. You can then open images in said editors from within ACDSee.
  1. In Manage mode, click Tools | External Editors | Configure Editors.
  2. In the Configure Photo Editors dialog box, click Add...
  3. Browse to the image editing application.
  4. Select the executable file of the editing application. For example, PhotoEditor.exe.
  5. Click Open.
  6. In the Name field, type a name for the editor, and then click OK.
  7. Click OK.
You can set a keyboard shortcut to open this external editor quickly.
  1. Choose Tools | Customize Shortcuts...
  2. In the Customize Shortcuts dialog box, choose Tools from the Category drop-down menu.
  3. From the Commands menu, select Default External Editor.
  4. Enter a shortcut key into the Enter new shortcut key field.
  5. Click Assign and close the dialog box.
Your new keyboard shortcut has been set. Now all you need to do is select the image in Manage mode that you want to open in the external editor, then press the keyboard shortcut you assigned.
You can also add the external editor to the Filelist toolbar in Manage mode.