You can do this with a script ...
#target photoshop
function main(){
var dlg=
"dialog{text:'Script Interface',bounds:[100,100,380,290],"+
"panel0:Panel{bounds:[10,10,270,180] , text:'' ,properties:{borderStyle:'etched',su1PanelCoordinates:true},"+
"title:StaticText{bounds:[60,10,220,40] , text:'Tile Picture' ,properties:{scrolling:undefined,multiline:undefined}},"+
"panel1:Panel{bounds:[10,40,250,130] , text:'' ,properties:{borderStyle:'etched',su1PanelCoordinates:true},"+
"statictext1:StaticText{bounds:[10,10,111,30] , text:'Accross' ,properties:{scrolling:undefined,multiline:undefined}},"+
"statictext2:StaticText{bounds:[140,10,230,27] , text:'Down' ,properties:{scrolling:undefined,multiline:undefined}},"+
"across:DropDownList{bounds:[10,30,100,50]},"+
"down:DropDownList{bounds:[140,30,230,50]}},"+
"button0:Button{bounds:[10,140,110,160] , text:'Ok' },"+
"button1:Button{bounds:[150,140,250,160] , text:'Cancel' }}};"
var win = new Window(dlg,'Tile Picture');
if(version.substr(0,version.indexOf('.'))>9){
win.panel0.title.graphics.font = ScriptUI.newFont("Arial","BOLD",20);
g = win.graphics;
var myBrush = g.newBrush(g.BrushType.SOLID_COLOR, [1.00, 1.00, 1.00, 1]);
g.backgroundColor = myBrush;
var myPen =g.newPen (g.PenType.SOLID_COLOR, [1.00, 0.00, 0.00, 1],lineWidth=1);
}
win.center();
for(var i=1;i<41;i++){
win.panel0.panel1.across.add ('item', i);
win.panel0.panel1.down.add ('item', i);
}
win.panel0.panel1.across.selection=0;
win.panel0.panel1.down.selection=0;
var done = false;
while (!done) {
var x = win.show();
if (x == 0 || x == 2) {
win.canceled = true;
done = true;
} else if (x == 1) {
done = true;
{
if(!documents.length)return;
var startRulerUnits = preferences.rulerUnits;
preferences.rulerUnits = Units.PIXELS;
doc = app.activeDocument;
app.displayDialogs = DialogModes.NO;
var tilesAcross = parseInt(win.panel0.panel1.across.selection.index)+1;
var tilesDown =parseInt(win.panel0.panel1.down.selection.index)+1;
var tileWidth = parseInt(doc.width/tilesAcross);
var tileHeight = parseInt(doc.height/tilesDown);
ProcessFiles(tilesDown,tilesAcross,tileWidth,tileHeight);
app.preferences.rulerUnits = startRulerUnits;
}
}
}
}
main();
function ProcessFiles(Down,Across,offsetX,offsetY){
activeDocument.flatten();
TLX = 0; TLY = 0; TRX = offsetX; TRY = 0;
BRX = offsetX; BRY = offsetY; BLX = 0; BLY = offsetY;
for(var a = 0; a < Down; a++){
for(var i = 0;i <Across; i++){
doc.activeLayer = doc.artLayers.getByName("Background");
doc.selection.select([[TLX,TLY],[TRX,TRY],[BRX,BRY],[BLX,BLY]], SelectionType.REPLACE, 0, false);
executeAction(charIDToTypeID( "CpTL" ), undefined, DialogModes.NO );
doc.selection.deselect();
doc.activeLayer = doc.artLayers.getByName("Background");
TLX = offsetX * (i+1) ; TRX = TLX + offsetX; BRX = TRX; BLX = TLX;
}
TLX = 0; TLY = offsetY * (a +1); TRX = offsetX; TRY = offsetY * (a +1);
BRX = offsetX; BRY = TRY + offsetY; BLX = 0; BLY = (offsetY * (a +1)+offsetY);
}
}
