The easiest way, that I know, in K 1.5 to add an image to a message is to use the image add button:
. Click the button, enter the URL of the image in the URL text box and press Insert.
However, if you mean "what's the easiest way to upload an image on my PC and incorporate it into my message" that's a different question.
After you have used the "Select image file to attach" feature, located below the message editor, to browse your PC for the image that you want to upload, press the [img] button. This will insert a [img/] placeholder in your message. When you submit your message, the placeholder will be replaced by the actual image.
I think that should answer "what's the easiest way" question. However, your other question, about image
size, is more difficult to answer.
Kunena automatically adjusts the size of images used in messages so that they won't break your site template if they're too big. In other words, if an image is physically too wide it will be displayed smaller so that it will fit within the boundaries of the forum template. You can use any image format, as long as it you have allowed it to be uploaded by you Joomla website and it can be rendered by your web browser.
Does this information answer your questions? :
One final point:
Support
category, where you posted your question, is intended to be used to address problems with the unmodified core
Kunena software - not to ask how to change it or to discuss problems people might encounter with third-party developed features - or to report defects that you are have found - not to ask questions about how to use the software normally or how to use Joomla normally. We give these support topics higher priority than other topics, particularly around the time when there is a new release of
Kunena (as there was only a couple of days ago), in order to ensure that defects are not passed on to other users. I have moved your topic to a more appropriate section.