The easiest way (for me, at least) is to use the "Manage Attachments" button and just upload the picture that way. No worry about having to resize the pictures, etc. Upload it, BF thumbnails it in your post, and you're good to go.
you'll need to resize your pics to less than 100kb, which is 640x480 for most pictures and some 800x600 pics if you want to use the attachment function for BF.
you can use a free and fast image managing program like FastStone to bulk resize pics.
I either use Photobucket, or if needed, just host the images from my website. As for resizing, I've not had to do a large batch, but the GIMP works quite well in this department, and allows for fine-tuning of the JPEG when saving it.