To upload an image to your dataform you need to use a file field.
You display the image using %%form_raw{field}%%, not %%form_data{field}%%.
As with images on normal pages you can set parameters like a float or the width. For example:
[[f<image %%form_raw{field}%% width="150px"]]
You can also combine data forms with the image box snippet created by Timothy Foster which will allow you to easily add a header, caption, a float left or float-right, specify the width and add a link for the image. In the data form use a file field for the image, a text field for the header and a text field for the caption.
Then to display them, above the ==== separator use the following snippet code with %%form_raw{field}%% for the image and %%form_data{field}%% for each of the header and caption. You do not need to have a value in each parameter line of the snippet code. An example of how it would look is below.
[[include :snippets:image
|image=%%form_raw{field}%%
|width=150px
|float=right
|heading=%%form_data{field}%%
|caption=%%form_data{field}%%
|link=*%%form_data{bandwebsite}%%
]]
On normal wiki pages you can upload an image to the page. This is not the case with data forms. When you upload an image using a data form, it places the image on its own page in the file category and the page is called the name of the image. So for example if your user presses the browse button in the data form and uploads an image called queen.jpg, that image is saved on the page http://yoursite.wikidot.com/file:queen
Although the filecategory is used by default for images, you can change the category the images on a data form are saved in. To do this use the category attribute as follows:
[[form]]
fields:
eventimage:
label: Image
type: file
category: eventimages
[[/form]]
If you had specified this in the data form structure in your live template before uploading the queen.jpg image it would have saved it at http://yoursite.wikidot.com/eventimages:queen
If you don't upload an image to a file field in a data form, older browsers like IE8 will show a box with a red x or similar. This doesn't look good and makes it seem that a mistake has been made. So, instead you can display a default image which will be displayed instead. This could be a blank image or a general image related to the site. If an image is uploaded to the field in the data form then that image will be used instead..
It needs a css module in the live template and this example also uses the image box snippet from http://snippets.wikidot.com/code:image-box to display the relevant image:
[!-- Following CSS module is needed for the default image code below --]
[[module CSS]]
.form-image-default%%form_raw{bandimage}%%{ display: block !important; }
.form-image%%form_raw{bandimage}%%{ display: none !important; }
[[/module]]
[[div class="form-image"]]
[[include :snippets:image
|image=%%form_raw{bandimage}%%
|width=150px
|float=right
|heading=
|caption=%%form_data{band_caption}%%
|link=%%form_data{band_link}%%
]]
[[/div]]
[[div class="form-image-default" style="display: none;"]]
[[include :snippets:image
|image=/css:theme/blankjpg
|width=150px
|float=right
|heading=
|caption=
|link=
]]
[[/div]]