You can use any number of image hosting services that are available on the Web, and many of them are for free (but sometimes with "catches").
Choosing one can be a problem because in recent months at least two such services that were widely used by forum posters have either closed ("Hostthenpost" seems to have vanished in the last few days) or gone completely subscription only ("Photobucket"). The lesson here is always keep a backup copy of any image you post on these services.
What happens next, how to paste an image into a post in these forums, will depend on which hosting service you use. The method is basically the same but the actual process or steps can differ between the hosts.
Also, do not use Google Photos or Google Docs as your image hosting service. One poster did unsuccessfully use Google Photos for posting images in these forums last year in an experiment. Because of the limitations Google puts on its hosted images, they are only visible for a few hours when posted on other web services.