(im using xp home) easiest way i found which dosent require any other software is:
get the image you want to upload right click on it and select open with, paint.
then in paint click image then select stretch/skew, then where it says horizontal type in say 90% and where it says vertical also type 90%
then click ok. save image as new one then right click on new image thumbnail in the containing folder to get properties which will then show the file size.
if its still to large to upload to uk420 then just go through the above process but type 95% for horizontal and vertical settings, re save and check. i just tried a 4.59mb file and changing the size in paint to 90% horizontal and vertical and then saving that as a new image has reduced the file size to 556kb so still a little way to go with the resize but if i was to try it again resising to 95% of the saved image size then it most probably will be under the 350k mark or whatever the limit is.
I found this keeps the zoom in function and keeps the image large in posts.
Click to view attachment This image was 4mb a bit of shufflin l8r and its like 277kb or something and it has kept all the detail and zoom function. The last of my pot of gold for those wondering. and it was lovely! been w/o for months now:((