I have assumed that you have created the new entity using the Advanced Customization Wizard.  You can download the latest version of the wizard here: https://community.sagecrm.com/partner_community/m/sage_crm_downloads/default.aspx.

I have also assumed that when the entity was created the option to include child documents was selected.

Our custom entity therefore has the ability to upload images to its 'library'.

I then made sure there was an option for 'Image' in the libr_type field, this is to allow me to indicate that the uploaded file is an image. I then added a dummy field into the project table e.g. proj_projectimage. I made the field read-only using the Field Level Security. I included the field in the default project summary screen - ProjectNewEntry.

Then I added the code to the create script of the 'dummy' field.

var strJump = Values("J");
re = /projectnew/i; 
r = strJump.search(re) 
if (r==-1)
{
var SID
var strPath = CRM.URL(220);
//the system action 220 is for the person summary screen
var arrayFullKeys = strPath.split("?");
//arrayFullKeys[0] contains path up like "/crm/CRM.dll/Do"
var arrayKeys = arrayFullKeys[1].split("&");
for (var i=0;i<arrayKeys.length;i++)
{
var arrayValue = arrayKeys[i].split("=");
if (arrayValue[0].toLowerCase()== "sid")
{
SID = arrayValue[1];
}
}
//This is used in pages which are linked to from list custom jump.

var intRecordId = Values("proj_projectid");
var myRecord = CRM.FindRecord('library',"libr_type='Image' and libr_projectid ="+intRecordId);
var pictureURL = arrayFullKeys[0]+"/";
if (!myRecord.eof)
{
//pictureURL += custom_sysparams.parm_value;
pictureURL += myRecord.Libr_FilePath;
pictureURL += myRecord.Libr_FileName;
pictureURL += "?SID=";
pictureURL += SID;
pictureURL += "&Act=1282&Mode=0&FileName=";
pictureURL += myRecord.Libr_FilePath;
pictureURL += myRecord.Libr_FileName;
Caption = "<img border=1 width=80 src='"+pictureURL+"'>";
}
else
{
Caption = "No Image Available";
}
}
else
{
Hidden = true;
}

Note: Because of my earlier assumption that the code for the ASP pages was generated by the Advanced Customization Wizard I can use the fact that the summary page is called projectsummary.asp and the entry page is projectnew.asp. This makes it very easy for me to determine whether I am in looking at an existing record or whether I am in the process of creating one.

To change this for your own entities you will need to look through the code and see where I have referenced the project entity. Just change those references for your own and all should be well.