Sure, no problem!
If you go to create a new post, here for you, scroll down to the place you uploaded your image. If you were successful on uploading an image earlier, clicking on the "Browse All" button will bring up a list of all images and media you have uploaded.
If your picture is there, we can move onto the next step, if not, you need to click back on the "Upload" tab and upload the image again. If it is there, click on it and you will be taken to another screen.
That screen should have the name of the image and some options. By default, the "thumbnail" will be highlighted with a radio button under the "show" and "File" will be selected under the "Link To" section. If you want to have the smaller thumbnail in yuour post and have it linked to the full size image that you uploaded, you can click the "send to editor" button now.
If you want to change the options, just click on the ones you want and then click "Send to Editor".
It will send the image into your blog post entry above it and you can continue on writing until you have your complete post!
Does that help?