Hi Jessica,
If you are using Media Library to store your image and you use the "Media gallery" webpart its default transformation show the file size data.
Here is the transformation code:
<%@ Register Src="~/CMSModules/MediaLibrary/Controls/LiveControls/MediaFilePreview.ascx" TagName="MediaFilePreview" TagPrefix="cc1" %>
<div class="mediaItem">
<table>
<tr>
<td class="mediaLibraryPhoto"><a href="<%# HTMLHelper.HTMLEncode(MediaLibraryFunctions.GetMediaFileDetailUrl(Eval("FileID"))) %>" title="<%# ResHelper.GetString(Convert.ToString(Eval("FileDescription"))) %>">
<cc1:MediaFilePreview ID="filePreview" runat="server" maxsidesize="117" width="117" height="117" /></a>
</td>
</tr>
<tr>
<td class="mediaLibraryDescription">
<a href="<%# HTMLHelper.HTMLEncode(MediaLibraryFunctions.GetMediaFileDetailUrl(Eval("FileID"))) %>" title="<%# ResHelper.GetString(Convert.ToString(Eval("FileDescription"))) %>">
<%# HTMLEncode(LimitLength(GetNotEmpty("FileTitle;FileName"), 18, "...")) %>
</a><br/>
Size: <%# DataHelper.GetSizeString(ValidationHelper.GetLong(Eval("FileSize"), 0)) %><br/>
Uploaded: <%# ((DateTime)Eval("FileCreatedWhen")).ToString("M/d/yyyy") %>
</td>
</tr>
</table>
<div class="mediaItemBottom"></div>
</div>
If you are not using Media Library it is still possible.
Have a great day