In order to upload a picture, you will need to click "Add New Entry", which can be found on the black bar at the top of the screen on the main website. If you're using the mobile browser view, the "Add New Entry" view is on the menu, and is labelled "Upload" on the apps.
From here click on the "Choose a photo"
Your browser will access your device's files and from here you will need to select your image.
You will then be asked if you would like to write a title and description and tag the image. You can also identify where it was taken on a map and choose to share it on Twitter. You can choose to share on Facebook on the next (confirmation) page.
Once you have finished this, simply click publish to finish uploading your Blip!
If you have any difficulty in uploading an image you can switch to the basic uploader by clicking on the link at the top of the upload page.