The easiest way I know how to reduce the size of a picture is to open it with the Paint program. Then go to "Image," then "Stretch/Skew." You can adjust the size by whatever percentage you want. I usually start with 50%. If that is still too big, undo it and try 25 or 35%. But you always want to undo it first before trying another size because if you reduce it to 50%, then 25% (of that 50%), you will lose a lot of the resolution. Save the reduced picture to your computer (I use 50% in the filename, to keep it separate) then upload from there.
Hope this helps.
I have windows 8 and when I open a picture I do so in Paint. While in paint under the HOME tab go to "resize" on the tool bar. A little window pops up with percentage already selected. Change this to PIXELS. Take the largest value of the horizontal or vertical and select a number lower than what it already (I usually start with half value) is and click on OK. By selecting one value Paint will automatically reduce the size and keep the ratio of the picture the same. Save the picture and if it is still to large reduce it again.