Make sure resolution is the same and then that sizes are appropriate. First, size your base image.
Target dpi
For viewing on web page, most practical: 72dpi and actual size you want it seen.
For printing in class, 200 dpi
For commercial printing, 300 dpi

Make resolution of images you are adding the same as the base image. Reduce or enlarge images you are adding to the size you will want them to be or slightly larger Choose one of several ways to move images onto the base image.

To move the whole image:
Drag the image icon from the Layers Panel onto the base document.
Select All, Copy and Paste onto base document.
To move just part of the picture:
Select tge area you want with a lasso or marquee tool and either
(1) Copy it and then paste it onto the base document or
(2) Use the Selection Tool (black arrow) to drag the selection from one document to the other

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