I assume you're using the WYSIWYG editor and using the inline image uploader?
If you upload an image and WP creates a thumbnail, then it will automatically link to the full-size version. It sounds as though some of your text is getting caught up in the link.
The best advice I can offer is to type your text first and add the image afterwards making sure there is a space before the start of the text.
Or open the HTML editor (click on the HTML icon from the WYSIWYG editor bar) and edit the actual link. Make sure to close the link just after the image tag and remove the end tag from the text.
Hope that helps.