The easiest and most flexible way overall, IMO, is to open an account on
Photobucket.com and upload your photos there. Their uploader will automagically resize your photos to meet their requirements (which allow considerably higher resolution than BCF does). Then you can insert a either links, like
https://i258.photobucket.com/albums/hh260/TR3driver/DSCF0013.jpg
or IMG codes
<div class="ubbcode-block"><div class="ubbcode-header">Code:</div><div class="ubbcode-body ubbcode-pre" ><pre>
</pre>[/QUOTE] into your BCF post. The above IMG code will appear as
when others view your post.
This technique also saves room on Basil's server, since the photos are actually stored over on Photobucket.
But if you want to upload directly, you need to find a tool to resize them. Personally, I am fond of
https://www.irfanview.com/ which is free and seems to work quite well. It won't let you specify the output size directly, but makes it very easy to convert to a particular resolution, and 640x480 will almost always fit. It also makes it easy to crop away unneeded parts of the photo, touch up colors, improve contrast and so on.