You can use the code to resize.
To give an example using an image I just made for a game; at full size it's too big to be of any practical use:
That's using the plain image tag:
[img]https://elliquiy.com/elluiki/images/1/13/Big_Jake.gif[/img]
To get it down to a manageable size I add either "height=" or "width=" after the "img" in the above tag followed by a number. To give an example:

[img height=600]https://elliquiy.com/elluiki/images/1/13/Big_Jake.gif[/img]
Or

[img width=200]https://elliquiy.com/elluiki/images/1/13/Big_Jake.gif[/img]
Note the code in both cases: I've added either a "height=X" or "width=X" after the "img" (but within the []). That dicates the image size. If you're feeling really crazy you can distort an image by altering both the height and width (if you only use one then the image remains in proportion):

[img height=300 width=1000]https://elliquiy.com/elluiki/images/1/13/Big_Jake.gif[/img]
It's best to work with a bigger image and downsize rather than a smaller one and upsize; you get a far higher quality.