If you have a photo editor that supports layers (such as Photoshop or Elements; there are others), open the flag picture. Separately in the same program, open the eagle. Select the eagle (all of the picture or just part) and copy it. Return to the flag, and paste. In some programs, you have to create a new layer before you can paste to it; in others, the Edit menu has a choice to paste a new layer. Sometimes, just the paste (CTRL + V) command automatically makes a new layer with the pasted information.
Next, set the transparency level of the eagle to 50%, or whatever else looks good.
You will have to merge the layers (may be called "Flatten Image") before you can save it as a JPG.