I've based this thread on using postimages.org file sharing, but this method should work for other other image file sharing sites.
Step 1
In your device browser go to https://postimages.org/ and click on 'Choose images', the other boxes can be used if you want a smaller resized image. Don't bother with the 'NoExpiration' box as that's only required if you want to automatically delete an image after a certain time period.
You can now tap on 'Choose an image' and pick an image from whatever folder you store images on your device, or from cloud storage such as One Drive by Microsoft that shows up on Android and Windows device folders if you have it installed and use it.

Step 2
Once the image has uploaded copy the highlighted link so it's ready to paste into the forum post you will make. The image will be large and will auto resize on the forum, it won't just be a small thumbnail picture!
At this point you have to create a new post on the forum, so you can shut down the postimage.org page as long as you're sure the link has been copied and is ready to be pasted. The copied link should still copy into your forum post even if you close the postimage.org page. Or you can keep the postimage.org tab open and open another tab to the forum if you haven't already, whatever is easier for you!

step 3
Once on the forum and you're ready to upload the image link you saved from postimage.org into your post, open 'Quick Reply' then click on 'Full Editor & Preview' that can be seen at the bottom of 'Quick Reply'. You can then paste the image using the link you copied.

'Full Editor and Preview' shown below

Step 4
Click on the tab shown below and you will see the img brackets show up.

Step 5
Now paste the link you got from postimage.org in the centre of both sets of brackets as shown below, there should be no spaces in any of the text or it won't work! Notice the 'Preview' tab which you'll use next.

Step 6
Now you can check if the image has been correctly inserted by tapping on the 'Preview' tab as shown below. You can now add your text above or below the image to finish the post or add more images, check preview again and if all looks good tap on the 'Submit button' to post on the forum. It's then job done!

Once you get the knack of uploading images to the forum this way it doesn't take long at all.
