I think the maximum forum image size is 800 x 600.
It's fairly easy to resize images using the 'paint' facility on Microsoft. Here are the steps -
1) open the image
2) click on the three dots on the top right corner of the screen …
3) a drop down menu will appear. Click on 'Open with'
4) another drop down menu will open. Select 'Paint' under the 'Other options' list and 'OK'.
5) the image will open in the 'Paint' programme - probably quite large. On the top left of the screen you will see a small 'resize' option. Click that. Choose 'resize by pixels' and change the pixel number from whatever it currently is to 800 horizontal by 600 vertical and press OK
6) Save the image as a new 'save as' image name eg, 'image 1 small'
7) You will find the new image in your gallery usually next to the original image. Upload as normal.
Hope this helps!