Forum posting sites are for sharing your knowledge to others related to your expertise. Form platforms should be used for sharing knowledge. For example this forum is SEO knowledge sharing platform, same forums will be there in your niche,
To find relevant forum platform and share and get knowledge related to your business niche should be a purpose to register in forum sites.
Here is the trick to find forums in Google
Image sharing has nothing to do with SEO. It does not mean your search results will increase by sharing images. Yes, Image optimization will surely help to do better for crawlers. You can optimize images by compression image size, adding proper ALT tags etc. If you are creating meaningful images and sharing on social media where your niche people are, it can bring traffic to your website. You can try these popular platforms to share your images Instagram, Facebook, and Pinterest etc.