Paul,
For image resizing, there are little programs that you can download for free.
Once installed, to use them, you just right-click on one or more images and specify how you want them resized in the dialog that pops up. I usually downsize to 750x750 pixels for this forum. You may or may not get options about the compression.
There is no particular reason why wrapping an image in a PDF should change its pixel dimensions. More likely, it will change its compression characteristics. It all depends on the program used to produce the PDF. I advise keeping your images as JPEGs. As such, they will be more-readily editable, and their metadata is more likely to be retained intact.
The leaves of Acer negundo are indeed variable in outline. In general, when comparing leaves for identification purposes, consider also the pattern on the main veins, the undersides, the mattness/shininess of the surface, the relative length of the petiole, and peculiar features such as tufts of hairs in the vein axils or the nature of teeth on the edges.
Bark is also a character that needs to be used with care. In most trees, the bark changes character as the tree ages. Shagginess, for instance, may not develop until maturity.
Do look for old seeds on the ground. These stand a good chance of confirming the genus and maybe the species (Acer negundo has fairly unusual samara). If you can't find any seeds under your particular tree, are there other similar specimens nearby?...
Mike