I generally post mine to Flickr, then share from there. If you want to attach directly to a post, use the “Upload A File” button at the bottom of the reply area. The other option is to upload to your gallery, here, then link back in the thread post.
There is likely a "resize" option in whatever software you use. If you tell us what operating system (windows vs mac) you use and which software you use, probably someone can give you specific advice. For example, on any mac there's a tool called "Preview" and even that tool has a "resize" option.
If you resize your images so that they don't exceed 1000 (or 1024) pixels on the "long" edge, that's usually a pretty friendly size.
You can actually drag & drop an image directly into your post. But I usually post to Flickr and then use Flickr's "share" icon, and select the "BBCode" choice. Then just copy & paste the string to your post here and it will embed your image in the post.
e.g.
That opens this panel:
Just pick the "BBCode" option at the top, then select the size using the pull-down menu. That creates the link (the "[url=...") with the correct settings to embed the correct image and at the correct size. So just copy and paste the link to your post here.
You need to be aware of image resolution (image size) or how many pixels by how many pixels and image file size, in Mb of kb.
TPF has an image file size limit of about 2 Mb that will prevent an image from being iploaded.
There is no image size (resolution) limit, but the more image resolution (size) an image has the larger the file size tends to be.