dia is obsolete.  Not updated for 10 years now.
Use diagrams.net online app instead (choose Device as file source to use local files).

Limited subset of Gimp, available for Linux, Windows, plus.

Native files are XML, which are gzipped by default.
You can not distinguish gzipped from non-gzipped by file name, since both use
only the extension .dia.

External images are always stored as relative file paths, in .dia and .svg
formats.
Therefore external images must be transported with the diagram.

To import an external image as an Object, File/Open it, then cut it from new
document (CTRL+X) and paste into current document.

For Arrows, just draw lines.  They automatically get a tip.
Lots of nice settings, but unfortunately no way to assign a shape to the head,
only to the tip.

OBJECT PROPERTIES.  Select! the object then RMB.
It does you no good to RMB on top of any particular object, since this is not a
context-sensitive menu.

To edit Text objects, use the tool with icon of an I-like input cursor.

GOTCHA:  Before pasting object(s), make sure that no object(s) is selected.
This works:  Before pasting, select the pointer icon and click in the bg.
If pasting multiple copies of something, you must repeat this before each paste operation.

Polyline.  Create first segment then RMB > Add corner.