Retouching or altering images, will generally involve selecting parts of that image to either change, copy or remove pixels for whatever purpose. Photoshop has various tools and options for selecting an area of pixels and it is useful to become familiar with them.
The task below shows you how to create a montage, using two different images, and primarily using the lasso tool to select an irregular shape.
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Use this link to open the pebbles image.
This will be the background for our montage.
Step 2 - Open the butterfly image 
Use this link to download the butterfly image.
When you have it open, zoom in on the image, then click on the Polygonal Lasso tool (arrowed left). You will need to use the tiny 'arrow' in the corner of the lasso tool icon to show the three tool options (right).
Check at A that the New Selection icon is selected and at B that there is no Feather set.
Now you are ready to begin selecting the outline of the butterfly.
Important: If you are NOT using the Polygonal Lasso tool, hold down the ALT key and begin to trace the outline of the butterfly, tracing it by making a series of short lines, much as you would in a 'Dot to Dot' tracing.
You can see the effect and the lasso cursor in the screen shot (left).
This method is the only way to easily define the shape. Ordinarily the Lasso tool, if used 'freehand' (just by clicking and dragging) is difficult to use accurately and impossible to do accurately. With the ALT key held down though, you can use both techniques if you want to.
Don't bother with the antennae if that is too fiddly for you. Also don't worry about not being able to see the whole butterfly. Photoshop will 'scroll' automatically for you when you reach the edge of the frame.
When you have traced around the whole of the butterfly and reached the point where you started, double click to 'complete' the selection. You will see the outline as 'marching' dots (left).
Now use the menu command EDIT > COPY to copy the selected butterfly.
'Q: Why can't I use the 'Magic Wand' tool?' The Magic Wand selection tool only works on images with a clear difference in pixel colours between the object and the background. Therefore it would not work on this image (or for around 80% of most most images in fact).
Return to the pebbles image and use menu command EDIT > PASTE to paste the copied butterfly into the image. Photoshop should automatically paste it in to a new layer.
Save this new image as a new Photoshop format file and try the following additional manipulations:
a) Use the Selection tool (arrowed on the left) to move the butterfly image around and with it selected:
b) Use the EDIT > FREE TRANSFORM command to make it smaller by dragging the 'handles' that will be added to it. (Tip: hold down the SHIFT key as you do this to retain its proportions)
c) Rotate it - also by using the Freeform tool
d) Use copy and paste several times to add a small 'colony' of butterflies.
When you have finished your image, make an evidence print, with copies of the unretouched butterfly and pebbles images and your new image on the document, suitably captioned.