Tools > Advanced Tools > Graphics > Place Graphic

find and select your .png file

click where you want to place it on the page, you can then drag to move it or size it

From the Manual (not a bad idea to read it ...):

Graphics placed in Scroll View and Page View are assigned differently. Finale always assigns a graphic to a measure if you are in Scroll View when the graphic is placed. To place a page-assigned graphic, you must be in Page View. Both page- and measure-assigned graphics can be edited and repositioned in Page View. However, page-assigned graphics do not appear in Scroll View.

For information regarding the relationship between graphics in the score and linked parts, see Graphics in linked parts.

When exporting graphics, Finale exports entire pages of music, or a selected region of music. To export a selected area of a page, you must be in Page View.